The present invention relates to a steam generation method. Especially, in the technological field of assisted recovery of hydrocarbons, the invention relates to a method for generating steam for applications of gravity drainage assisted by steam injection, known as SAGD method (Steam-Assisted Gravity Drainage).
The in-situ exploitation of oil deposits of bituminous sands by the SAGD technique currently uses either natural circulation steam boilers with a separating flask or forced circulation steam generators of the OTSG (Once Through Steam Generators)-type. Both types of steam generators require a feed of high quality water. However, the water recovered in the SAGD method is far from meeting the high quality criteria and has to undergo an expensive treatment.
This treatment consists in numerous filtering, sweetening and ion exchange apparatuses such that the feed water comprises as little particles and minerals as possible.
The main reason which requires a high level of water treatment derives from the fact that the heating of water produces a boiling which concentrates solids and salts dissolved in the liquid phase. When the concentration exceeds a critical threshold, solids are deposited on the exchange surfaces, inhibiting thereby cooling of said surfaces and causing a local overheating of the metal. This overheating possibly causes a metal breaking, which can cause significant shutdown and non-production periods.
The very large amounts of water required for the SAGD method coupled to environmental and governmental constraints require the major part of used water to be recycled. This thereby results in a gradual concentration of dissolved solids into water, which amplifies the above-described deposition problem. Although the OTSG-type steam generators withstand the presence of dissolved solids better than boilers with flask, they can eventually be subjected to the fouling effect.
Currently, the water treatment is estimated to monopolize up to 70% of capital expenditure and 20% of operating costs of a steam generating plant in the SAGD method.
In the known steam generation methods, steam and droplets of water are generated at the boiler outlet. The steam part represents the so-called steam quality. It is low in the known methods, being of the order of 75%.